1. Field of the Invention
The present invention concerns in general improving the quality of reception in a radio receiver and especially an adaptive equalization in order to tune the radio receiver on such a frequency that the quality of the received signal is the best possible.
2. Description of the Prior Art
The receiving of radio broadcast suffers from interference caused by the frequency shift and broadening, other radio broadcasts on the same and adjacent channels and other electromagnetic radiation sources. In the receiver the interferences show themselves as noise and signal distortions. Especially in the digital radio broadcast, where the modulation is based on a digital signal to be transmitted, the interferences produce bit errors, that is, the receiver does not interpret the bits transferred by the signal correctly. A digital receiver can analyze after the decoding the data received by it, if there are redundancy or certain checksums attached to it, and if necessary, abandon the incorrect parts of the data. This causes, however, ineffectiveness of the data transfer, so that it would be better, if the receiver could be tuned so that it could already in the receiving phase decrease the influence of the interferences. The problem is complicated by the fact that the interferences are mostly temporally variable, particularly in cases of mobile terminal devices.
In addition to external interference factors, the need for frequency tuning is influenced by the internal factors of the receiver, like aging of the components, manufacturing tolerances, temperature drift, intermodulation frequencies and so forth.
Known in the art there is a method called AFC (Automatic Frequency Control), wherein the receiver includes an adjustable mixer frequency oscillator, a testing circuit for measuring the received signal power and a feedback loop that adjusts the oscillator so that mixing to the intermediate frequency always happens by using the mixer frequency that produces the strongest possible signal power. A disadvantage of this method is that the strongest received signal power doesn""t always correspond to the best signal quality e.g. in a situation where the receiver, in addition to the desired signal, also captures an undesirable transmission on the same or a very close frequency. The tuned frequency sets itself so that attenuation of the intermediate frequency path is the smallest possible. If the transmission characteristics of the filters are not symmetrical (in other words, the attenuation minimum in the middle of the pass band), the intermediate frequency is tuned aside, even though the receiver would not receive anything else but the intended transmission. The tolerances of the filters and their adjustment circuits can cause even a significant lateral deviation, whereby the received signal is distorted, that is, its quality is deteriorated.
Another known method is to use a frequency synthesizer, in other words, a very accurately tunable mixer oscillator, the mixer frequency produced by which is some multiple of the basic frequency produced by the crystal oscillator. As the mixer frequency remains accurately the same, the receiving filter can be constructed to have a very narrow band, whereby the influence of the interferences propagating on other frequencies is decreased. A disadvantage of this method is that if the transmitter or the receiver or some factor reflecting the signal moves, the frequency of the desired signal does not keep stable but changes, whereby it can be driven outside the narrow pass band of the filter. Additionally, the tolerances of the filters and matching circuits can cause that the attenuation of the signal path is not at its minimum on the nominal frequency. In that case the signal/noise ratio suffers, that is, the quality of the signal is deteriorated. The method is also sensitive to the changes happening to the characteristics of the components in the course of the time.
One method of prior art is the AGC (Automatic Gain Control) where the signal to be received by the receiver is attenuated in the prestages of the device in cases where the level of the signal to be received is too high. The tuning of the receiver is not changed in the AGC method. In addition, in a double super heterodyne receiver with two intermediate frequencies, it is known to tune the mixer frequency received from the first local oscillator aside, whereby the first intermediate frequency signal sets itself aside. The second mixer frequency received from the local oscillator is also tuned aside, but in such a way that the second intermediate frequency signal is moved in another direction with respect to the nominal frequency than the first intermediate signal. In that case the effective bandwidth through the intermediate frequency phases is narrowed. The narrowing and widening of the band can be based on the level of the received signal or on the signal/noise ratio. The adjustment of the bandwidth and the detection of interferences demand, however, separate components, whereby the production costs are increased.
An object of the present invention is to provide a method and a device for improving the quality of the digital radio reception compared with the prior art. A further object of the invention is also to provide a method and a device in which the quality improving effect is not significantly dependent on the age of the components or manufacturing tolerances. A still further object of the invention is, that the method and device in accordance with it are suitable for large-scale serial production and have reasonable production costs.
The objects of the invention are attained by measuring the quality of the digital signal decoded in the receiver, and by controlling the selecting of the frequency used for the intermediate frequency mixing so that the frequency producing the best quality is continuously or at least with regular intervals looked for.
The method in accordance with the present invention is characterized in that it consists of the phases where
a certain data channel of a received signal, said data channel transferring data in
a digital form, is demodulated and decoded,
the quality of said data in a digital form is measured on a certain frequency and
at least on one further frequency, and
decision is made based on the results of the quality measurement,
whether the radio receiver part of the receiving device will be further kept tuned on the same frequency,
whether it will be tuned on some new frequency, or
whether the correctness will be further measured on other frequencies before a decision is made of returning the radio receiver.
The present invention also concerns a device, in which the above described method is used.
A device in accordance with the present invention comprising
a radio receiver to be tuned step by step on certain frequencies for receiving and demodulating radio signals,
a data decoder for decoding digital data from a demodulated signal, and
a controller for processing digital data and for tuning said radio receiver, is characterized in that it further comprises a measuring element for measuring the quality from the decoded digital data and for transmitting a parameter describing said quality to said controller.
In the method in accordance with the present invention, the seeking of the optimal frequency is based on the measurement describing the quality of the received and decoded signal. In case of a digital signal, the most natural way to measure the signal quality is to monitor the quantity of the bit errors occurring in it. The invention does not limit the scale used for the quality evaluation. Most simply, the signal can be either of good or bad quality, whereby there are two steps in the scale. In scales with more steps the signal can be e.g. totally correct, a little faulty, much faulty or useless.
The receiver demodulates and decodes the digital signal received by it and forwards it to the quality measuring element. The quality is measured, in addition to the tuned frequency respectively, also on at least one frequency below and/or above it. In case the signal quality measurement gives bad results, the quality measurements can also be extended farther from the tuned frequency. The receiver selects a new tuned frequency based on certain algorithms, after which the measuring cycle is started from the beginning. The receiver can repeat the measurements continuously or it can perform the measurement at certain intervals.
Although there are different tuning methods based on the quality measurement of the signal, known from the processing of analog signals (among others from the transfer of video and audio signals), the present invention is not obvious based on them, because, for the first, the quality measurement of an analog signal requires equipment of a laboratory level, and secondly, because in that case a special test signal should be transmitted in the signal, in order to make the quality determination possible. Also systems, in which both analog and digital data is transmitted simultaneously, cannot be based on the tuning optimization based on the data signal only, because the demands e.g. for analog video signal transfer are totally different from those for the data transfer.